Real estate is precious. Some creatures defend territories extending over several kilometres, but when Jayne Yack (Carleton University, Canada) encountered miniature newborn warty birch caterpillars (Falcaria bilineata) she wondered if she might have discovered one of the world's smallest, and youngest, territorial critters. 'We had noticed that tiny warty birch caterpillars produced vibrations', says Yack, who first encountered the feisty little creatures in 2008. She also noticed that the tiny caterpillars – 1 to 2 mm long – reside in solitude on birch leaves, making her speculate whether they are warning off intruders with the vibrations they transmit through their leaves from their leaf tip residences. Yack and her colleagues report in Journal of Experimental Biology that the minute caterpillars are territorial, warning off intruders with vibrations produced by scraping their rear ends along the leaf, in addition to beating the leaf tip with their thorax, to defend the world's smallest territory.
To find out more about the tiny caterpillars , Yack, Sarah Matheson, Leonardo Turchen and Emilie Mauduit (also from Carleton University) first needed to collect female two-lined hooktip moths (F. bilineata), ready to lay eggs on leaves and pieces of paper in the lab, in order to collect the caterpillar youngsters and transfer them to the centre of a single birch leaf as soon as they emerged from the eggs, to find out which region of the leaf they prefer to inhabit. A day later, almost 90% of the youngsters had wriggled to the tip of the leaf to set up home.
Having determined that the youngsters establish a territory, the team filmed the tiny caterpillars as they scouted their minute home range, rested and fed, rarely straying more than a few body lengths from the tip. And from time to time the youngsters beat the leaf with their head and thorax, producing a percussive tone, in addition to dragging oar-shaped hairs on their rear along the leaf, producing a buzzing scraping sound, while simultaneously striking the leaf with the head end of their bodies. 'The sounds produced by these caterpillars are not audible humans, so we had to use specialized equipment to pick up the vibrations', says Yack.
Once she was certain that the teeny creatures were territorial, Yack and her colleagues wondered how the youngsters would react when an intruder came calling. Placing an interloper near the centre of an occupied leaf, the team sat back and watched each encounter unfold. Sometimes the intruder turned tail and headed off in search of another leaf after approaching the resident, which had been frantically scraping at the leaf sending urgent warnings as the trespasser advanced. However, on other occasions the occupant secured themselves to the leaf with a silken thread before flinging themselves over the edge – after a protracted period of leaf scraping – as the threat approached, leaving themselves dangling safely beneath the leaf. Yet the caterpillars never came to physical blows, biting or thrashing each other. The outcome of each contest depended entirely on the warning vibrations transmitted through the leaf. Yack suspects that the youngsters might prefer this more diplomatic form of conflict resolution to ensure they both survive.
But why defend a leaf tip? Yack suggests that the tip may be more succulent than other parts of the leaf, offering the resident better nutrition, in addition to providing the occupant with a springboard for escape, if necessary. She also suggests that intruders are unable to approach residents ensconced at the tip from behind, limiting their opportunities for attack, and could even allow the tiny caterpillars to appear larger than they are as the leaf might amplify the resident's warning signals. Whatever the reason, tiny newborn warty birch caterpillars could tip the scales as the tiniest kings in their leaf tip castles
